If they killed Pentangeli, what would Roth gain? It sure wouldn't take Michael out of the picture. And why would they plant heroin on him?
The only way it works is for it to be a fake hit. They didn't NEED to know a cop would show up. Roth engineered the whole thing. Roth could have simply had a cop waiting up the street until Pentangeli and Rosatos went inside.

All the Rosatos would need to know is that they need to keep pentangeli alive, mention Michael Corleone and plant drugs on him. Roth would handle the police/FBI end of the deal.

Hell, it might even be better if the cops DID kill the unsuspecting Rosato brothers so Roth could have a Jew take over the territory. Either way, Roth's gain would only come through eliminating Michael- either by murder or by sending him to jail for a long time. Hence the deal for Pentangeli to roll over on Michael and Questadt to steer the hearing against him.